1. Basic Chemistry Chapter 11 Gases Chapter 11 Lecture
Fourth Edition
Chapter Gases
Partial Pressures
Learning Goal Use Dalton’s Law of partial pressures to calculate the total pressure of a mixture of gases.

2. Partial Pressure
The partial pressure of a gas
is the pressure of each gas in a mixture
is the pressure that gas would exert if it were by itself in the container

3. Dalton’s law of partial pressures states that the total pressure
depends on the total number of gas particles, not on the types of particles
exerted by a gas mixture is the sum of the partial pressures of those gases
PT = P1 + P2 + P
Total pressure of
the gas mixture
= Sum of the partial pressures of
the gases in the mixture

4. Total Pressure Equal Sum of Partial Pressures
The total pressure of two gases is the sum of their partial pressures.

5. Air, Mixture of Gases
The air you breathe is a mixture of gases.
A typical mixture of gases in the air includes: N2, O2, CO2, Ar, and H2O.

6. Air, Mixture of Gases

7. Learning Check
A scuba tank contains O2 with a pressure of atm and He at 855 mmHg. What is the total pressure in mmHg in the tank?

8. Solution
A scuba tank contains O2 with a pressure of atm and He at 855 mmHg. What is the total pressure in mmHg in the tank?
Step 1 Write the equation for the sum of partial pressures.

9. Solution
A scuba tank contains O2 with a pressure of atm and He at 855 mmHg. What is the total pressure in mmHg in the tank?
Step 2 Rearrange the equation to solve for the unknown pressure.

10. Solution
A scuba tank contains O2 with a pressure of atm and He at 855 mmHg. What is the total pressure in mmHg in the tank?
Step 3 Substitute known pressures into the equation and calculate the unknown pressure.

11. Collecting Gas over Water
A gas produced in the laboratory
usually contains water vapor
PT = Pwater + Pgas
has a partial pressure obtained by subtracting the vapor pressure of water from the PT

12. Collecting Gas over Water
Magnesium (Mg) reacts with HCl, to produce H2 gas.
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
As the H2 gas is produced, it displaces some of the water in the container.
The gas collected will be a mixture of H2 and water vapor.

13. Collecting Gas over Water
We use the vapor pressure of water at the experimental temperature to determine the amount of dry H2 gas produced in the reaction.
Figure 11.9 A gas from a reaction is collected by bubbling through water. Due to evaporation of water, the total pressure is equal to the partial pressure of the gas and the vapor pressure of water.

14. Guide, Collecting Gas over Water

15. Learning Check
When magnesium reacts with HCl, a volume of L of H2 gas is collected over water at 26 °C. The vapor pressure of water at 26 °C is 25 mmHg. If the total pressure is 752 mmHg, how many moles of H2 gas were collected?
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)

16. Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
Solution
In the following reaction L sample of H2 gas is collected over water at 26 °C. If the total pressure is 752 mmHg, how many moles of H2 gas were collected?
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
Step 1 Obtain the vapor pressure of water.
The vapor pressure of water at 26 °C is mmHg.

17. Solution
In the following reaction L sample of H2 gas is collected over water at 26 °C. If the total pressure is 752 mmHg, how many moles of H2 gas were collected?
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
Step 2 Subtract the vapor pressure from the total P of the gas mixture to give the partial pressure of needed gas.

18. Solution
In the following reaction L sample of H2 gas is collected over water at 26 °C. If the total pressure is 752 mmHg, how many moles of H2 gas were collected?
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
Step 3 Use the ideal gas law to convert Pgas to moles or grams of gas collected.

19. Solution
Step 3 Use the ideal gas law to convert Pgas to moles or grams of gas collected.
Solve for moles of H2 gas by placing the partial pressure of H2 (727 mmHg), volume of gas container (0.355 L), temperature (26 °C = 299 K), and R, using mmHg into the ideal gas equation.

20. Chemistry Link to Health
During breathing, an exchange of gases occurs in which
oxygen from the air diffuses into the lungs and into the blood
carbon dioxide produced in the cells is carried to the lungs to be exhaled

21. Chemistry Link to Health
In Table 11.8, partial pressures are given for the gases in air that we inhale (inspired air), air in the alveoli, and air that we exhale (expired air).
The partial pressure of water vapor increases within the lungs because the vapor pressure of water is 47 mmHg at body temperature.

22. Chemistry Link to Health

23. Chemistry Link to Health

24. Concept Map